Problem: Before the previous stop there were 20 people riding on a train. 2 people ended up getting off at the stop. How many people are riding the train now?
Explanation: The number riding the train now is the difference between the number who were riding and the number who got off at the previous stop. The difference is $20 - 2$ people on the train. $20 - 2 = 18$ people are on the train.